The racemic compounds of formula I wherein R.sub.3 stands for hydrogen are known. They are described the first time by Lajos Szabo et al. (Tetrahedron Letters 39, pages 3737-3747). The optically active compounds wherein R.sub.3 stands for hydrogen and the racemic and optically active compounds wherein R.sub.3 represents --CH.sub.2 OH are new.
The racemic or optically active diester derivatives of formula I are very important intermediates for the synthesis of pharmaceutically active compounds of eburnane skeleton such as vincamine and vincamone, and the apovincaminic acid esters, e.g. Cavinton.